A disk drive comprises a rotating disk and a head actuated over the disk to magnetically write to and read data from the disk. The head may be connected to a distal end of an actuator arm that is rotated about a pivot to position the head radially over the disk. The disk may comprise a plurality of radially spaced data tracks, where each data track is partitioned into data sectors. The disk may also comprise a plurality of angularly spaced servo wedges, where each servo wedge may include a servo sector for each data track providing embedded servo information for the track. The servo information may include a pattern of alternating magnetic transitions (servo burst), which can be read from the disk by the head and processed by a servo controller to determine the position of the head over the disk. The servo sectors may be written on the disk using an external servo writer. Alternatively, a disk drive may self-write the servo sectors on the disk using any suitable technique, such as propagation or while following seed tracks, such as spiral tracks.
Regardless of how the servo sectors are written on the disk, errors may occur in the servo writing process resulting in the servo sectors defining eccentric tracks that deviate from concentric circular tracks. In addition, eccentricity may be caused by the disk being clamped to the spindle motor off center and/or the disk slipping after being clamped to the spindle motor. The eccentricity may be referred to as repeatable runout (RRO) since the position error relative to the head repeats each time the disk rotates.
To compensate for repeatable runout associated with a track, RRO compensation values may be generated for the track and written in the servo sectors for the track. The RRO compensation values written in the servo sectors (also referred to as wedge RRO (WRRO)) for a particular track may later be read from the disk and processed by a servo controller to adjust the head position accordingly to compensate for the repeatable runout.